Timber will always move to a certain extent through thermal expansion, deflection, vibration or differential movement. In addition, where movement of the top surface of the timber is restricted by means of solidly fixed tiles, the movement within the body of the timber will cause stresses to build resulting in the tiled surface or the grout cracking. There are a variety of covering systems available that allow wooden backgrounds to move without affecting the tiles on the surface while maintaining a strong bond, but in all cases timber must be properly secured before further work takes place.
It is essential to ensure that all wooden backgrounds are fully sealed against moisture. Some types of marine ply have a dye in them that will cause a reddish /orange stain to be drawn through adhesive which can bleed through to the surface of natural stone or blemish the appearance of glass tiles. This can only appear if moisture is able to absorb into the wood surface. Seal plywood with a two part primer in dry areas, or Waterproof Coating on intermittently wet wall areas. It is essential to use a flexible adhesive when tiling onto primed timber surfaces.
Chipboard or hardboard is not an acceptable background for tiling. If chipboard or hardboard has been installed or is in situ it will need to be replaced or covered with a suitable material for tiling. Any plywood flooring should be a minimum of 18mm thick, cross laid over existing floors and screw fixed at a maximum of 300mm apart. Floors prepared with timber sheet material should be flat, level and in a sound, stable condition. Floorboards must be secured with twin screw fixings as they cross each joist.
The timber floor should be primed with a polymer primer before fixing tilebacker boards, in a brick pattern, with a 3-4mm solid bed of pourable flexible adhesive. The boards should be screw fixed at a rate of 5 fixings per square metre, remembering to use washers to prevent the screw heads tearing into the board. In wet areas the screw and washer fixings should be covered with a waterproof jointing tape. The joints between the tilebacker boards should be strengthened with a scrim jointing tape in dry areas or Waterproof Jointing Tape in wet areas. Both tapes are fixed in a fine bed of flexible adhesive to provide a suitable seal.
In areas over 20 square metres scrim matting should be fixed over the whole area in a fine layer of flexible adhesive. This will not be necessary if a mesh backed underfloor heating is to be installed.
Ditra matting performs a similar function to tilebacker board without using any mechanical fixings. The wooden floor should be secured in the same manner prior to fitting tilebacker board. Ditra matting is attached to the floor with the fleece side facing down using Ardex AF200 adhesive. Any joins between rows of matting should be covered with Kerdi-Keba band fixed with flexible adhesive. Once the joints have cured and hardened an even layer of adhesive should be spread over the Ditra matting to smooth it ready for tile fixing. Once the smoothing layer has been completed and is dry, the tiles can be fixed using a thin solid bed of adhesive. It is important to note that the chosen tiles must be 50x50mm or larger when using Ditra Matting. If smaller mosaic tiles are to be used on the floor then the timber will need to be prepared with tilebacker board. In all cases the final adhesive depth over Ditra matting must not exceed 10mm.